Chlorination of Saturated Hydrocarbons WILHELM HIRSCHBIND Great Western Division, The Dow Chemical Company, Pittsburg, Calv. -#
Chlorination of saturated hydrocarbons, particularly those of low molecular weight, came into technically and commercially successful operation in 1935. Unlike the long-established practice of chlorination of unsaturates such as ethylene and benzene, the chlorinationof methane and homologs is much more difficult because of the necessity of activating the chlorine by either photochemical or thermal means. Both methods are used commercially today, and each has its advantages and disadvantages.
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HLORINATION of saturated hydrocarbons was carried out commercially for the first time by Sharples Solvent Corporation, in 1929, in the chlorination of pentane. Ayres (8) describes two possible methods of chlorination of pentane as follows: (1) Chlorination below 100' C. in either vapor or liquid phase requires either light or catalyst. The yields of primary chlorides are low. (2) Chlorination above 200' C. in the vapor phase (liquid phase requires very high pressures) can be accomplished without light or catalyst and gives the highest yields of primary chlorides. The latter method was followed and chlorine was fed continuously into a stream of hot pentane vapor, resulting largely in the formation of primary chlorides. Chlorination of methane, ethane, and propane was not commercially practiced until some years later. The strongly exothermic character of the chlorination reaction presents great difficulties of control and leads only too easily t o the formation of carbon and hydrogen chloride. An extensive investigation of the chlorination of natural gas was carried out by the U. S. Bureau of Mines (9) but did not lead to a single commercial application. The investigators could have obviated some of their difficulties had thev seDarated the constituents of the natural gas and experimented with pure materials-a luxury which physicists seldom practice but which chemists can ill afford to miss. Even the analysis of the many chlorinated hydrocarbons formed in the chlorination of natural gas presented great difficulties a t that time, before the advent of infrared spectroscopic analyses, to say nothing of the even greater difficulties in large scale fractionation.
The photochemical method is influenced by traces of impurities causing inhibition of reaction through chain termination. The thermal method requires temperatures of at least 250' C. for starting, which, together with the large heat of reaction, demands close temperature control. A continuous manufacturing process for chlorination of methane in which all chlorinated methanes are produced is described. Novel processes for manufacture of perchloroethylene and hexachloroethane are also presented.
Method of Activation. Both thwmal and photochemical activation are practiced commercially. For thermal activation it is necessary that the gas mixture containing the chlorine be heated above 250" to 300' C. to obtain an appreciable rate of reaction. The mercury arc lamp is the usual source of light for photochemical activation, since a large percentage of its radiation lies in the region of 3000 to 5000 A. units. Wave lengths much shorter than this are not transmitted by Pyrex, whereas the longer wave lengths are not absorbed appreciably by chlorine. I n general, any chlorine reaction with methane, ethane, or propane produces polysubstituted products as well as monosubstituted products, even when the part reacted is quite small. Thus, the product distribution for gas phase thermal chlorination of methane as shown in Figure 1, taken from some unpublished data of this company which are in fairly close agreement with those of McBee and Hass (IO), shows that it is impractical to obtain methyl chloride alone from methane.
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CHEMISTRY OF THE REACTION
Chain Reaction. Available evidence indicates that the reaction of methane, ethane, and propane and their chlorination substitution products with chlorine is a chain reaction. The chains are initiated by the formation of chlorine atoms from the dissociation of chlorine molecules by thermal or photochemical energy. These reactive atoms then react with a molecule t o form hydrogen chloride and an organic free radical. This radical is believed to react with an undissociated chlorine molecule regenerating a chlorine atom to perpetuate the chain and a t the same time producing a molecule of organic chloride. Thus, several hundred, or even thousand, chlorine molecules may react as a result of a single activation reaction, provided that steps are taken to prevent termination of the chain by impurities which react preferentially with the active chain carriers.
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Figure 1. Thermal Chlorination of Methane
Very similar product distribution results from photochemical chlorination in both gas and liquid phase. Product distribution for liquid phase chlorination of ethylene dichloride is shown in Figure 2. Conditions for best economic yields of each of the chlorinated products can readily be derived from the curves. Heat of Reaction. A very important factor in the chlorination is the removal of the high heat of reaction. Although accurate figures are not available on each of the successive steps of chlorina2749
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tion, the published data, as well as theoretical considerations of the bond energies involved, indicate that a figure of 650 B.t.u. per pound of chlorine reacted is applicable to the formation of practically any saturated chlorinated compound from a saturated hydrocarbon.
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Photochemical ChIorination of Ethylene Dichloride
Chlorination Methods. PHOTOCH~MICAL CHLORINATION. Photochemical methods have not frequently been applied commercially because of high equipment and operating costs. The reason why they are economically feasible for chlorination reactions lies in the high quantum yield. A typical figure for plant conditions is froni 10 to 30 pounds of chlorine per hour per 400-watt H-1 mercury arc lamp, depending on specific conditions of chlorine concentration, hydrocarbons, and temperatures. Calculations show that the above rates correspond to at least 30 to 90 molecules of chlorine reacted per quantum of light emitted. Photochemical chlorination is applicable to both gas and liquid phase chlorinations. The chlorination of intermediate products such as methylene chloride, chloroform, and ethylene dichloride by the liquid phase method is practical and efficient. Hydrocarbons and gaseous chlorination products can also be chlorinated in liquid phase by bubbling them together with chlorine through the illuminated liquid products, but the gas phase method is preferable. Liquid phase photochemical chlorination of methane results primarily in production of carbon tetrachloride; gas phase chlorination lends itself to the production of intermediates. THERMAL CHLORINATION. The thermal method of chlorination requires temperatures above 250 a C., a t which level the dissociation into chlorine atoms becomes apprcciable. The high heat of reaction can be utilized in this method for preheating and the resulting high temperature favors rapid rates and high reactor capacities. Moreover, the method lends itself to the production of unsaturated compounds through the combination of chlorination, dechlorination, and dehydrochlorination. The comparative advantages of each method are: Photochemical 1. Absence of unsaturated compounds as impurities in products 2 . Freedom from tar and carbon formation 3. Rapid starting method because time-consuming preheating is not recluired Thermal 1. Not affected by inhibitors 2 . Not affected bv electrical disturbances and Dower fluctuations; no electrical maintenance 3. High capacity, low installation cost 4. Complete reaction of chlorine 5. Substantial yields of unsaturated compounds such as perchloroethylene and chlorinated propenes
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Specific Types of Chlorination Reactors. From the above discussion it is apparent that three main types of reactors can be used-namely, liquid phase photochemical, gas phase photochemical, and thermal. LIQUID PHASE PHOTOCHEMICAL, The heat of reaction can conveniently be removed by cooling coils installed in the liquid and by reflux condensers which condense the vaporized liquid and return it to the tank. Chlorine and hydrocarbon are introduced through perforated bubbler pipe encircling the light source. ID order to maintain high rates and to avoid by-passing unchlorinated feed liquids, it is desirable to baffle the flow so that all liquid must pass close to the light source. For example, at a chlorine content of 0.1% measurements indicate that 90% of the active light is absorbed within a distance of 5 inches. TOOhigh A chlorine concentration in the liquid can reduce the over-all rate of chlorination by absorbing the light before i t reaches the more remote sections of the tank. I n the extreme case, for instance, calculations indicate that 90% of the incident light is absorbed by only 0.006 inch of liquid chlorine. The chlorine concentration is easily controlled by regulating the temperature of the liquid in the reactor. The liquid phase photochemical chlorination is particularly susceptible to inhibitors, which must be carefully removed. GAS PHASE PHOTOCHEMICAL. The temperature rise in gas phase reactors in gencral is taken up largely by the specific heat of the gases, thus limiting the amount of chlorine in one reactor to approximately 20%. Subsequent additions must be made if further chlorination is desired. The reactors should be large enough to utilize most of the available light--at least several feet in diameter. I n genpral, it is difficult to reduce the chlorine content of the exit gas to less than 2%. Although gas phase reactore are susceptible to inhibition by oxygen and other impurities, the problem is less serious than in liquid phase. The percentage of chlorine in the feed gas must be carefully controlled, since even 8 temporary excess can lead to carbonization and explosions. GAS PHASE THERMAL. The teniperature rise is controlled by the specific heat of the gases, Diluents in form of carbon tetrachloride, nitrogen, hydrochloric acid, OF water vapor can be provided if necessary. Complete reaction of the chlorine is very easily attained in presence of excess of hydrocarbon. It is necessary that the inlet flow be mixed with hot reacted gases to propagate the reaction unless the feed is preheated above approximately 250 to 300 C. Heat capacity in the vessel is useful in stabilizing the temperature during momentary upsets. The reactor may be started by electrical heat, a carbon are, or by heating with a hnt; gas strcam. O
DEVELOPMEKT O F PROCESS
Work on chlorination of methane in these laboratories began ia, 1832. Because of the large quantity of heat liberated in the chlorination reaction, chlorinating agents which evolved heat in their formation and consequently absorbed heat during chlorination were used. Such compounds are sulfuryl chloride, phosgene, and nitrosyl chloride. Sulfuryl chloride was the only material used which reduced the heat liberated in the chlorination of methane to about one half. The method, subject to a patent by McKee and Salls ( I I ) , was investigated but was abandoned shortly when it became evident that the cost involved in the recovery and re-use of the large quantities of sulfuryl chloride made the commercial application doubtful. The work then was continued with gas phase thermal chlorination based on heat transfer through solid walls as heat transfer agents (3) Artificial graphite is one of the few materials which withstand the action of chlorine and hydrogen chloride at reaotion temperatures; in addition it is a good conductor of heat. Reactors were constructed in which methane and chlorine pasra through narrow spaces which gradually widen as the reaction progresses. However, in spite of the good heat conductivity of
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1949 CH,,
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Flow Sheet of Gas Purification Process
the graphite, the porosity of the material caused carbon formation, and after a pilot plant period of some duration the method was abandoned. Subsequently photochemical chlorination was tested starting with liquid phase reactors and converting methylene chloride to chloroform and chloroform to carbon tetrachloride (4). The principle of this chlorination has been outlined in the previous part of this paper, and the results were satisfactory as to chlorination rate per lamp unit. When methane was chlorinated in liquid phase by bubbling a mixture of chlorine and methane into liquid carbon tetrachloride, it was found that the rates per lamp were considerably lower. Because of the necessary high light input to attack the methane, the intermediates chlorinate so rapidly that the end product consisted largely of carbon tetrachloride. However, a considerable amount of methyl chloride, methylene chloride, and chloroform was formed in the gas space above the liquid; this led to the development and perfection of the gas phase reactors for the initial photochemical chlorination of methane.
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the chlorinated hydrocarbons formed are absorbed in a mixture of carbon tetrachloride and chloroform, while methane and hydrochloric acid escape from the top of the column. The liquid effluent is then fed to a stripping column, where it is freed from the lowerchlorinated products and chlorine. The stream from the top of the absorption column, termed major gas, contains largely hydrochloric acid and excess methane, whereas the stream escaping from the stripping column, termed minor gas, contains the lowerchlorinated hydrocarbons, chlorine, and a substantial quantity of hydrochloric acid because of the latter's solubility in the carbon tetrachloride-chloroform mixture, The high boiling fraction of the chlorinated hydrocarbons remains in the circulating liquid; it is separated later by distillation. The absorption of hydrochloric acid from both streams is the subject of the next step ( l a ) . 3. The minor stream, containing hydrochloric acid and lowerchlorinated hydrocarbons, passes through an adiabatic rolumn fed with hot water. The chlorinated hydrocarbons, saturated with water vapor but containing small quantities of hydrochloric acid and chlorine, escape from the top, while hot dilute acid, containing less than 15% hydrogen chloride, flows from the bottom of the column through an intercooler into the top of a second water-cooled absorption colum? built of Karbate. This column is fed with the major stream from the absorption column containing the bulk of the hydrogen chloride and forming commercial 20 O B6. hydrochloric acid. The escaping methane, saturated with water vapor and containing some hydrogen chloride, goes first through a neutralizing column, then through a sulfuric acid drying column, and passee into the suction of a compressor to be recycled into the reactor system. 4. The minor stream, consisting of the lower-chlorinated hydrocarbons contaminated with hydrochloric acid and some
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Methyl Chloride, Methylene Chloride, Chloroform, and Carbon Tetrachloride. The commercial application of the method dependedthen on the ability t o separate the hydrochloric acid formed from the different chlorinated products and to fractionate the chlorinated hydrocarbons from each other. The separation of the hydrochloric acid from the chlorinated hydrocarbon stream in presence of excess methane and chlorine presented some difficulties which were solved by adopting a unique absorption system in which full strength hydrochloric acid of 20" B6. (31.60j0)hydrochloric acid was recovered ( 1 ) . A unit incorporating this process was designed in 1940 and built and operated in 1941. It consisted of the following steps illustrated by Figure 3 (gas purification) and Figure 4 (reaction and distillation system) : 1. For reasons already explained methane was carefully purified, not only from the higher hydrocarbons such as ethane and propane but also from other impurities such as nitrogen. For this purpose a conventional absorption system, with two absorption columns in series, was employed. The first one removes ethane and heavier hydrocarbons; the second separates the methane from nitrogen. The methane is released in pure form from the absorption oil in a low pressure flash tank. 2. The pure methane from the low pressure flash tank passes into gas phase chlorinators of either the photochemical or thermal type, where it is chlorinated stepwise to produce methyl chloride, methylene chloride, and some chloroform. The chlorinated gas stream, containing excess methane, hydrochloric acid, and small amounts of chlorine, goes first into an absorption column, where
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Hexachloroethane is a product of direct addition chlorination of perchloroethylene and has, in the past, been recovered by crystallizing it from perchloroethylene so!utions folIowed by centrifuging and drying. During the war, two new processes were developed: onc, by the Hooker Electrochemical Company, was practiced in three eastern plants; and the other by Dorv, Tl-hich was operated a t the Texas and Great Western Divisions of The Dow Chemical Company. Dow's method was based on the premise that distillation is the simplest means of producing a pure product (8). Since the triple point of hexachloroethane lies a t 187" C. a t 1100 mm., the distillation must be carried out under prcssure. The proces? is shown in Figure 6 (hexachloroethane process), .._I..
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Flow Sheet of Perchloroethylene Process
chlorine, goes also through a neutralizing and sulfuric acid drying column and then into the suction of a compressor to the distillation system. The first distillation column, operating under pressure, separates methyl chloride from methylene chloride. The second distillation column, operating a t lower pressure, separates most of the methylene chloride from its mixture with chloroform. 5. The chlorinated hvdrocarbons fiom the bottom of the methylene chloride column, together with the accumulated material from the stripping column, are fed to a liquid phase reactor, where the remaining methylene chloride is converted into chloroform and some carbon tetrachloride. T h e effluent from the reactor overflows into a storage tank, forming the first break in this so-far toiallv continuous %em. T h e liquid from the storage tank is fed into a topping column, wliere unconverted methylene chloride is taken off as overhead and recycled through the liquid phase chlorinator, while the bottom effluent is fed into the chloroform refining still, wheie U.S.P. grade chloroform is distilled off. T h e bottom stream from the chloroform still is fed into a second liquid phase reactor, where it is continuously chlorinated to carbon tetrachloride to he fed into a column where the refined grade of carbon tetrachloride is produced. Perchloroethylene. During the war, presswe a as exerted on Dow t o produce large quantities of hexachloroethane for the armed forces; this required { h a t Dow produce its own perchloroethylene. Several methods for manufacture of this latter compound were in use, the principal one being the chlorination of acetylene to symmetrical tetrachloroethane, which was afterward converted by dehydrohalogenation and chlorination to perchloroethylene. It was also known that thermal cracking of earb'on tetrachloride produces perchloroethylene and chlorine, and this mcthod was practiced during the ~ a r . I n developing a method for DOWthe principle was employed of utilizing the heat of the chlorination reaction to crack the Chlorinated hydrocarbons to perchloroethylene with simultaneous formation of varying amounts of carbon tetrachloride. The raw materials can be hydrocarbons and partly chlorinated hydrocarbons of the methane, ethane, propane, or even higher series (6-7). The process is shonn in Figure 5 (perchloroethylene process). The hydrocarbon is mixed with diluent vapors of chlorinated hydrocarbons and fed into a thermal reactor, where it is chlorinated and cracked simultaneously. The vapors pass through a quench column from which hydrogen chloride escapes as overhead and is absorbed, while the side stream, consisting in the main of carbon tetrachloride and perchloroethylene, goes to fractionatlon. I n the first column carbon tetrachloride is distilled as overhead, while the bottoms flow into a second column from which perchloroethylene is distilled off. Hexachloroethane. The perchloroethylene produced during the war furnished the raw material for hexachloroethane production.
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Flow Sheet of Hexachloroethane Process
Perchloroethylene is continuously chlorinated in a liquid phase chlorinator to a point where the chlorination rate decreases perceptibly. It overflows into a storage tank from which it is fed into a column operating under pressure. Excess perchloroethylene is distilled overhead and recycled. A side stream of pure molten hexachloroethane is taken off and solidified by a unique method. The liquid molten hexachloroethane is vaporized and fed through a nozzle between two water-cooled rolls, where i t is solidified and flaked. Since the heavy vapors must lie in a pool between the two rolls, the design of the nozzle is all-important to prevent their surging and consequent escape. LITERATURE CITED
(1) Allen, G. L., Heitz, R. G., and Henderson, G. L., U. S. Patent 2,402,978 (July 2, 1946). (2) A y e s , E. E., IXD. ENG.CHEM., 21, 899-904 (1929). (3) Bender, H., U. S. Patents 2,074,885 (March23, 1937) ; 2,089,937 (Aug. 17, 1937); 2,170,801 (Aug. 29, 1940). (4) I b i d . , 2,200,254-5 @lay 14, 1940). (5) Brown, T. E., and Davis, C. W., I b i d . , 2,377,669 (June 5, 1945). (6) Davis, C. W., Dirstine, P. H., and Brown, W.E., I b i d . , 2,442,323 (May 5,1948). (7) Heitz, R. G., and Brown, W.E., I b i d . , 2,442,324. (8) Heits, R. G., and Oldemhaw, C. F., I b i d . , 2,445,526 (July 20, 1948). (9) Jones, G. W., Allison, V. C., and Meighan, hf. H., U . S. B u r . Mines, Tech. Paper 255 (1921). (10) McBee, E. T., Hass, H. B., Neher, C. M., a n d Strickland, H.. I N D . ENG. CHEW., 34,296-300 (1942). (11) McKee, R. H., andSalls, C. M., U. S. Patent 1,765,601 (June24, 1930). (12) Oldershaw, C. F., Simenson, L. O., Brown, T., and Radcliffe, F., Chem. Eng. Progress, 43,376 (1947). RECEIVED March 18, 1949. Presented before the Division of Industrial and Engineering Chemistry a t the 115th Meeting of the i h E R X C A N C x E M l C A h SOCIETY,San Francisco, Calli.
